1. Field of the Invention
The present invention is directed to an apparatus and method for the continuous casting of molten metal and, in particular, molten steel. The present invention may include the molten metal flowing into a continuous casting mold for casting thin slabs. The molten metal may flow through a submerged nozzle (casting tube), the nozzle including at least one lateral opening directed to a side wall of the continuous casting mold and at least one bottom opening directed in a casting direction.
2. Discussion of Background Information
The process of continuously casting steel with a submerged nozzle has been disclosed in, e.g., U.S. Pat. Nos. 3,888,294 and 3,991,815, the disclosures of which are incorporated by reference herein in their entireties. Submerged nozzles with lateral openings and at least one bottom opening have generally been utilized for the production of thin metal slabs and for enabling high casting rates. However, the cross-section of these slabs is small.
In the conventional continuous casting process utilizing a submerged nozzle, the process may be divided into a start-up phase and a normal operating phase. An exemplary discussion of the phases of the continuous casting process may be found, for example, in U.S. Pat. No. 4,787,438 the disclosure of which is incorporated by reference herein in its entirety. During the start-up phase, the casting rates must increase from zero to a nominal (set) rate corresponding to the normal operating phase. In general, the start-up phase may last for approximately one to two minutes. Further, difficulties similar to those occurring during the start-up phase of the casting process also occur following a replacement or exchange of the submerged nozzle. Thus, during the start-up phase, the submerged nozzle does not operate within the set range of casting speed or the set rate for which it is designed.
Consequently, molten metal flows which occur during the start-up phase are completely different from the flow occurring during normal operation. Thus, the amount of the molten metal directed toward the casting level during the start-up phase is much less than desired, which results in a danger of top crust or bridge formation. This danger is much greater during start-up than during continuous casting in the normal operating phase.
Another drawback occurs during the start of casting in a conventional continuous casting process utilizing a submerged nozzle with a bottom opening to enable a vertically directed downward stream of molten metal immediately toward a dummy bar head. The dummy bar head is utilized for closing the bottom end of a permanent continuous casting mold and is covered with cooling chips to avoid forming a welded joint between the strand and the dummy bar head. The downwardly directed casting stream washes the cooling chips away from the dummy bar head which can result in difficulty separating the cast strand from the dummy bar head. Additional discussion regarding the dummy bar (starter bar) head may be found in U.S. Pat. No. 5,456,307 and EP 0,114,309, the disclosures of which are incorporated by reference herein in their entireties.
Further drawbacks in the conventional continuous casting process arise during the exchange of the tundish (i.e., the vessel) which contains the molten metal and from where the molten metal flows through the submerged nozzle. The tundish exchange is generally made either to change the quality of the molten metal or to effect repair work.